1. Field of the Invention
The present invention relates to a test procedure for testing friction components of an automatic transmission.
2. Background Art
Automatic transmissions use wet friction components for automatically shifting planetary gear sets. Examples of wet friction components used in automatic transmissions include band brakes and clutch packs. In the design of automatic transmission systems, friction components must be tested to assure adequate transmission performance. If friction components are not properly designed or matched to an automatic transmission, shifting may be rough, jerky, or inefficient. In addition, the reliability and durability of friction components may be reduced.
Referring to FIG. 1, a standard inertia-absorption-type test stand such as an industry-standard Society of Automotive Engineers #2 (SAE#2) test stand is shown. SAE#2 test stands have been widely used to evaluate friction component engagement behavior. However, engagement conditions on SAE#2 test stands are inconsistent with the shifting kinematics of many automatic transmission systems. For example, in a typical up-shift that involves two friction components, sliding speed of the on-coming friction component slightly increases or remains nearly constant during an initial phase of the engagement (i.e., the torque phase of the shifting) even after it develops engagement torque. The slip speed starts decreasing toward zero only after the off-going friction component disengages at the beginning of the inertia phase. However, in a conventional inertia-absorption test, the slip speed starts decreasing as soon as the engagement torque starts rising. Due to this inconsistent slip speed profile, conventional test methods used on friction component test stands fail to accurately capture realistic engagement effects such as hydrodynamic effects on engagement torque.
During shifting, friction component engagement torque primarily determines the level of automatic transmission output shaft torque. The engagement characteristics of friction components have a direct impact on shift quality. Engagement behavior varies widely under different operating conditions. Oil and friction material characteristics may also have a significant impact on the engagement process. During automatic transmission system development, it is important to accurately characterize friction component engagement behavior under all operating conditions.
Referring again to FIG. 1, the SAE#2 inertia-absorption-type test stand is illustrated. A friction component 10 to be tested (i.e., either a band brake system as illustrated in FIG. 1 or a plate clutch assembly) is mounted on a drive shaft 12 that is connected to at least one inertia wheel 14 and a driving motor 16. One part of the friction component 10a is connected to the motor 16 while the other part of the friction component 10b is grounded to a housing 18. The frictional interface is lubricated with oil to simulate operation in a wet friction component environment.
Referring to FIG. 2, a flow diagram illustrating a control procedure for a SAE#2 friction component engagement test stand is provided that shows how the test stand is used in a typical prior art test protocol. The test control system initializes control variables at a block 20 wherein the system is initialized at t=0 with V(0)=0 and F(0)=0. A motor speed or drive shaft speed V(t) and an engagement force F(t) are measured at a block 22. Other variables may also be measured during the test, including a friction component engagement torque T(t). The driving motor 16 first raises the drive shaft speed V(t) to a prescribed value Vtarget as shown in a block 24 through either an open-loop control or a closed-loop control using measurements of V(t) as a feedback signal. When V(t) reaches its target value of Vtarget, as shown in a block 28, the drive motor 16 is decoupled from the drive shaft 12 and the friction component 10 at a block 30. At a block 32, the test control system controls the engagement force F(t) to follow a prescribed force profile Ftarget(t) through either an open-loop control or a closed-loop control using measurements of F(t) as a feedback signal. As soon as the friction component 10 engagement torque develops, the drive shaft 12 slows down in response to engagement torque and the system's mechanical inertia. When the drive shaft speed V(t) reaches zero at a block 34, the test cycle is complete. This system fails to create a test condition that corresponds to the torque phase of actual automatic transmission shifting.
Referring now to FIG. 3, test data measured on the test stand of FIG. 1 is illustrated in accordance with the prior art test procedure. The motor 16 rotates one part of the friction component 10a to achieve a target sliding speed Vtarget prior to the test, as shown by reference numeral 36. When engagement is commanded at a time t0, an actuator 19 applies an engagement force pneumatically or hydraulically to the friction component 10. The engagement force profile is shown by reference numeral 37. The sliding speed remains at about a value Vtarget while the actuator 19 strokes against its return mechanism. At or near the end of stroking at a time t0+ts, the motor 16 is powered off or de-coupled from the drive shaft 12. As soon as the engagement torque rises at t0+ts, the sliding speed starts decreasing. Engagement torque is shown by reference numeral 38. As engagement proceeds, the sliding speed decreases, approaching zero.
The prior art test methodology is limited to evaluating engagement behavior of automatic transmission shift events whose kinematics allow the sliding speed to drop as soon as engagement torque rises. In many automatic transmission shift events, the sliding speed actually increases or remains nearly constant during the torque phase even after friction component engagement torque develops. Unrealistic or erroneous engagement test results for a friction component may adversely impact application of the test results in automatic transmission design.
For the foregoing reasons, the present invention is directed to providing a method for testing friction component engagement behavior on friction component test stands to obtain results that are consistent with automatic transmission shift kinematics.